The instructions for making the cells in the human body are encoded in deoxyribonucleic acid (DNA). DNA is a long, ladder-shaped molecule, in which each corresponding rung is made up of a pair of interlocking units, called bases, that are designated by the four letters in the DNA alphabet—A, T, G and C. ‘A’ always pairs with ‘T’, and ‘G’ always pairs with ‘C’. The sequence of these four letters that make up an individual's DNA is referred to as the individual's genome.
The long molecules of DNA in cells are organized into pieces called chromosomes. Humans have 23 pairs of chromosomes. Other organisms have different numbers of pairs—for example, chimpanzees have 24 pairs. Chromosomes are further organized into short segments of DNA called genes. The different letters A, T, G, and C, which make up a gene, dictate how cells function and what traits to express by dictating what proteins the cells will make. Proteins do much of the work in the body's cells. Some proteins give cells their shape and structure. Others help cells carry out biological processes like digesting food or carrying oxygen in the blood. Using different combinations of the As, Cs, Ts and Gs, DNA creates the different proteins and regulates when and how they are turned on. Information about an individual's DNA sequence, including his or her genome or particular regions of the genome is referred to as genotypic information. Regions of a particular individual's genome can also be referred to as “DNA sequences.”
Each person has the same set of genes—about 20,000 in all. The differences between people come from slight variations in these genes. For example, it's not that a person with red hair has the “red hair gene” while a person with brown hair has the “brown hair gene.” Rather, all people have genes for hair color, and different versions of these genes, i.e. differences in the regions of the genome containing the gene, dictate whether someone will be a redhead or a brunette.
Variations in DNA sequences can generate biological variations between people by causing differences in the recipes for proteins that are written in genes. Those differences can in turn influence a variety of traits such as appearance, disease susceptibility or response to drugs. While some differences in the DNA sequences among individuals lead to differences in health or physical appearance, some variations in the DNA sequences among individuals seem to lead to no observable differences between people at all.
Every region of a person's autosomal (restricted to “non-sex” chromosomes) genome is represented by a pair of DNA sequences, one inherited from the mother and one from the father. Also, each child inherits virtually 50% of his or her DNA from the father and virtually 50% from the mother. The DNA inherited from the mother can be either inherited from the maternal grandmother or the maternal grandfather. The DNA inherited from the father can be inherited from the paternal grandfather or the paternal grandmother. Because each parent has two DNA sequences for every genomic region only one of which he or she will pass to each child, two siblings may inherit the same sequence from the father or the same sequence from the mother, the same sequence from both, or different sequences from each parent. As relationships become more distant, there is a lower likelihood that two individuals will have the same sequence. Because it is useful to know the similarity between two individuals' DNA, it would be useful to have a method for comparing the genotypic information of at least two individuals and displaying the comparison in a way that can be easy to understand.